TWI659672B - Method and apparatus for producing core-shell type metal nanoparticles - Google Patents

Abstract

An object of the present invention is to provide a method and an apparatus for producing core-shell metal nano particles having excellent productivity.

The present invention provides a method for manufacturing core-shell metal nano particles, which comprises the following steps: (a) introducing a first metal salt solution into a first flow path of a flow-type reaction device, and in the first flow path, The step of applying a plasma to the first metal salt solution to obtain a solution of metal nano particles containing the first metal; and (b) introducing the second metal salt solution into the second flow path of the flow-type reaction device to make it And a step of converging with the solution of the metal nanoparticle containing the first metal into a mixed solution, applying plasma to the mixed solution, and coating the metal nanoparticle of the first metal with the second metal.

Description

Method and device for manufacturing core-shell metal nano particles

The invention relates to a method and a device for manufacturing core-shell metal nano particles.

Metal nano particles are used not only as alloy particles for manufacturing thermoelectric conversion materials by sintering, but also as catalysts such as ternary catalysts, photocatalysts, and other functional powders.

As a conventional method for producing metal nano particles containing a plurality of metals, for example, a method of adding a reducing agent to a solution containing a plurality of metal compounds to precipitate metal nano particles.

For example, in a conventional method for producing composite metal nano particles of Bi and Te used as a thermoelectric conversion material, a reducing agent such as NaBH ₄ is added to a solution of a metal compound such as BiCl ₃ or TeCl ₄ . The composite metal nano particles of Bi and Te are precipitated.

In recent years, a so-called solution plasma method is known, which generates a plasma in a solution containing a metal compound and uses the reduction action of the plasma to precipitate metal nano particles.

For example, Patent Document 1 describes a method for producing metal oxide nano particles by generating a plasma in a solution containing a metal oxoacid. Further, Patent Document 2 describes a method for producing a metal nanoparticle having a particle diameter of 500 nm or less by generating a plasma in an aqueous solution of a metal salt.

In addition, a method for producing metal nano particles using a flow type reaction apparatus, such as a microreactor, is known.

For example, Patent Document 3 describes a method in which a hydrazine solution is mixed into an aqueous solution containing a metal salt in a microreactor to form a hydrazine complex, and the obtained hydrazine complex is borrowed Reduction from an alkali solution produces metal nano particles. In addition, Patent Document 4 describes a method for producing a raw material solution supplied into a microreactor by irradiating a single or plural energy beams of laser light, electromagnetic waves, particle beams, or ultrasonic waves. Metal nano particles.

As one of the forms of such metal nano particles, core-shell metal nano particles are known.

For example, Patent Document 5 describes a method in which nano particles of ZnO as a core are injected into a dispersant heated to a high temperature by using a so-called hot soap method, and as the precursor ₃ CoSb, CoSb ₃ by the coating to be produced ZnO core - shell metal nanoparticles.

[Prior technical literature] [Patent Literature]

[Patent Document 1] Japanese Patent Laid-Open No. 2011-195420

[Patent Document 2] Japanese Patent Laid-Open No. 2008-013810

[Patent Document 3] Japanese Patent Laid-Open No. 2013-108121

[Patent Document 4] Japanese Patent Laid-Open No. 2008-246394

[Patent Document 5] Japanese Patent Laid-Open No. 2005-294478

An object of the present invention is to provide a method and a device for manufacturing core-shell type metal nano particles that are excellent in productivity and have a high degree of freedom in designing the core and the shell.

However, in the conventional method of precipitating metal nano particles containing additives such as a reducing agent and a dispersant, impurity elements derived from such additives may remain in the metal nano particles. Here, the impurity element is generally removed by a process such as washing or thermal decomposition of the obtained metal nanoparticle, and it has been conventionally considered that the impurity element has been sufficiently removed by this.

For example, in the method described in Patent Document 5, the obtained core-shell metal nano particles are subjected to a thermal decomposition treatment to remove impurity elements.

However, the inventors of the present case found that it is difficult to completely remove the impurity element by processing such as washing and / or thermal decomposition, and therefore, the core-shell type metal nano particles are obtained by alloying them. alloy Traces of impurity elements remain in the particles, which may impair the thermoelectric conversion characteristics of the product or the characteristics of the catalyst function.

Here, the present invention also aims to provide core-shell type metal nano-particles that are extremely reduced in the possibility of impairing the thermoelectric conversion characteristics or characteristics of the catalyst function of the product, and alloy particles obtained by alloying them.

As a result of intensive research, the inventor of the present case has completed the present invention described below.

<1>. A method for manufacturing core-shell metal nano particles, comprising the following steps: (a) introducing a first metal salt solution into a first flow path of a flow-type reaction device, and in the first flow path, A step of applying a plasma to the first metal salt solution to obtain a solution of metal nano particles containing the first metal; and (b) introducing the second metal salt solution into the second flow path of the flow-type reaction device, The solution of the metal nanoparticle containing the first metal converges and becomes a mixed solution, and a plasma is applied to the mixed solution to cover the metal nanoparticle of the first metal with the second metal.

<2>. The method according to the aforementioned <1>, wherein the redox potential of the first metal is lower than the redox potential of the second metal.

<3>. The method according to the aforementioned <1>, wherein the first metal is Te and the second metal is Bi, or the first metal is Bi and the second metal is Te.

<4>. The method according to any one of the above <1> to <3>, wherein the equivalent area diameter when the cross-sectional area of the flow path is converted into a circle of the same area (equivalent diameter) is 1 μm to 10 mm.

<5>. A flow type reaction device comprising: a first flow path; a second flow path; and a third flow path. The third flow path is formed by converging the first flow path and the second flow path. The circuit has at least one plasma-generating electrode pair, and the third flow channel has at least one plasma-generating electrode pair.

<6>. The flow-type reaction device according to the aforementioned <5>, wherein the equivalent diameter of the flow path of the portion where the plasma is applied is 1 μm to 10 mm.

<7>. A core-shell metal nanoparticle obtained by a method including the following steps: (a) introducing a first metal salt solution into a first flow path of a flow-type reaction device, A step of applying a plasma to the first metal salt solution to obtain a solution containing metal nano particles of the first metal; and (b) introducing the second metal salt solution to the second flow path of the flow-type reaction device, A step of converging the solution with the metal nanoparticle containing the first metal into a mixed solution, applying a plasma to the mixed solution, and coating the metal nanoparticle with the first metal with the second metal;

<8>. An alloy particle obtained by alloying the core-shell type metal nanoparticle of the aforementioned <7>.

<9>. A thermoelectric conversion material, which is obtained by sintering the core-shell metal nanoparticle of the aforementioned <7> or the alloy particle of the aforementioned <8>.

According to the present invention, it is possible to provide an excellent productivity, and Core and shell design method and apparatus for manufacturing core-shell metal nano particles with high degree of freedom in design.

Further, according to the present invention, it is possible to provide core-shell type metal nano particles having extremely low possibility of impairing the characteristics of the product, and alloy particles obtained by alloying the particles.

10‧‧‧ Flow type reaction device

11‧‧‧First Stream

12‧‧‧Second stream

13‧‧‧ Third Stream

14a, 14b‧‧‧electrode pair

20‧‧‧ the first metal salt solution

30‧‧‧Second metal salt solution

40‧‧‧ Solution containing core-shell metal nano particles

50‧‧‧ batch reactor

51‧‧‧ raw material solution

52‧‧‧electrode pair

53‧‧‧ Stirrer

[Fig. 1] Fig. 1 is a schematic diagram showing an exemplary embodiment of a method of the present invention for manufacturing core-shell type metal nano particles and a flow-type reaction apparatus of the present invention.

[Fig. 2] Fig. 2 is a schematic diagram showing a method of Reference Example 1 for manufacturing core-shell metal nano particles.

FIG. 3 is a graph showing the time required until 12 g of core-shell metal nano particles are obtained in Examples 1 and 2, and Reference Example 1. FIG.

[Fig. 4] Fig. 4 shows (a) a scanning transmission electron microscope (STEM) image of a core-shell metal nanoparticle of Te-Bi manufactured according to Example 1 of the present invention, and (b) Curves from the analysis results of energy dispersive X-ray spectrometry (EDX).

5 is a (a) scanning transmission electron microscope (STEM) image showing a core-shell metal nanoparticle of Te-Bi manufactured according to Example 2 of the present invention, and (b) Curves from the analysis results of energy dispersive X-ray spectrometry (EDX).

FIG. 6 is a graph showing the concentration (ppm) of Na as an impurity element in the composite metal nano particles of Te and Bi in the comparative example and the alloy particles.

FIG. 7 is a transmission electron microscope (TEM) image of Au-Cu core-shell metal nanoparticle manufactured according to Reference Example 2. FIG.

8 is a transmission electron microscope (TEM) image of Au-Co core-shell metal nanoparticle manufactured according to Reference Example 3. FIG.

[Best Mode for Implementing Invention] "Manufacturing method of core-shell metal nano particles"

In the present invention, the core-shell type metal nanoparticle is a "shell of at least one layer" having a "core" and a covering core. The core may include at least a first metal, and the shell may include at least a second metal.

The method of the present invention for manufacturing core-shell type metal nano particles is a method for manufacturing core-shell type metal nano particles including the following steps: (a) introducing a first metal salt solution into a flow-type reaction device A first flow path, a step of applying a plasma to the first metal salt solution in the first flow path to obtain a solution of metal nano particles containing the first metal; and (b) introducing the second metal salt solution into a flow-type reaction The second flow path of the device is a step of converging with a solution containing metal nano particles of the first metal into a mixed solution, applying a plasma to the mixed solution, and coating the metal nano particles of the first metal with the second metal.

The solution plasma method is generally a method in which a plasma is generated in a solution containing metal ions, and the reduction of metal ions is performed by the reduction of the plasma to precipitate metal nano particles.

The reaction site of the solution plasma method is only between the electrodes that generate the plasma. Because the reaction site is small, the solution plasma method is generally considered to be inferior in productivity.

In contrast, the method of the present invention uses a flow-type reaction device and continuously applies a plasma to a raw material solution by using a solution plasma method to continuously produce core-shell metal nano particles. In addition, it is considered that the flow-type reaction apparatus of the present invention can be parallelized in a large scale.

Therefore, the productivity of the method of the present invention is more excellent than that of a solution plasma method using a batch method to produce core-shell metal nano particles.

In addition, the solution plasma method preferentially deposits a metal having a high redox potential (that is, a metal that is easily reduced) in accordance with the power of the applied plasma. Therefore, for example, when a solution containing two or more kinds of metal ions is used, in order to precipitate a metal having a low redox potential (that is, a metal that is not easily reduced), and a high-power plasma is used, the metal that is easily reduced is also Precipitation. Therefore, the conventional batch-type solution plasma method is generally considered to use a metal that is not easily reduced as a core and a metal that is easily reduced as a shell.

In contrast, in the method of the present invention, as the first metal, a metal that can be easily reduced can be selected and used as a core, and as the second metal, non-selected The easily-reducible metal is not only the shell, but the first metal may also be a non-reducible metal as the core, and the second metal may also be the easily-reducible metal as the shell.

Therefore, according to the method of the present invention, a beneficial effect that the so-called core and shell design freedom is high can be obtained.

This beneficial effect is further explained as follows.

The inventors of the present case have discovered that when sintering composite metal nano particles to obtain an alloy material (for example, a thermoelectric conversion material), relatively more metals that vaporize relatively may be vaporized, and the desired alloy composition may not be obtained.

Specifically, the present inventors have discovered that when sintering composite metal nano particles containing, for example, Bi and Te to obtain a thermoelectric conversion material, relatively more vaporized Te is relatively vaporized, and a desired alloy composition cannot be obtained. (For example, Bi ₂ Te ₃ ).

As a countermeasure, it is considered that the amount of more easily vaporizable metal is estimated by adding the more easily vaporizable metal, but the reduction of the yield should be avoided. In addition, since the amount of metal vaporization is not constant, a desired alloy composition cannot be obtained stably.

Specifically, when sintering a composite metal nanoparticle containing, for example, Bi and Te to obtain a thermoelectric conversion material, it is considered that more Te is placed in order to estimate the amount of Te that is more easily vaporized. Te is high in price, so the reduction in yield should be avoided. In addition, even if the allowable reduction in yield is not constant, the desired alloy composition (for example, Bi ₂ Te ₃ ) cannot be obtained stably because the vaporization amount of Te is not constant.

In contrast, by the method of the present invention, regardless of whether the metal is easily reduced or not, a metal that is easier to vaporize can be used as a core, and a metal that is less easy to vaporize can be used as a shell. The loss of metal is good, and the desired alloy composition can be obtained stably.

Specifically, when the composite metal nano particles containing Bi and Te are sintered by the method of the present invention to obtain a thermoelectric conversion material, Te which is easily reduced can be used as a core, and Bi which is not easily reduced can be used as a shell. Therefore, during the sintering, by coating the less vaporizable Te with the less vaporizable Bi, the loss of the easily vaporized Te during the sintering can be suppressed, so the yield is good, and the desired alloy composition can be obtained stably. (For example, Bi ₂ Te ₃ ).

In addition, the inventors of the present invention have found that metal nano particles are sintered by, for example, hermetic heating or hermetic pressure heating to obtain alloy materials of any shape. For example, when obtaining thermoelectric conversion materials, metals with lower melting points are High metals are melted first, and then melted out to a minute shape, for example, they may be melted out to the gap between the mold and the cover, and the desired alloy composition and / or uniform alloy composition cannot be obtained.

Specifically, the inventors have found that when sintering metal nano particles containing Bi and Te, for example, by closed heating or closed pressure heating to obtain a thermoelectric conversion material of any shape, the lower melting point of Bi is higher than the melting point. Higher Te melts first, and Bi melts out to the fine shape, for example, it will melt out to the gap between the mold and the cover, etc., and the desired alloy composition and / or uniform alloy composition (for example, Bi ₂ Te ₃ ).

In contrast, by the method of the present invention, regardless of whether the metal is easily reduced or not, a metal with a lower melting point can be used as a core, and a metal with a higher melting point can be used as a shell. Therefore, when sintering, the core metal with a low melting point can be melted first, and then the shell metal with a high melting point can be melted, so that the melting out of the metal with a low melting point can be reduced, and a uniform alloy composition can be obtained stably.

Specifically, by the method of the present invention, it is also possible to use Bi, which is not easily reduced, as a core, and Te, which is easily reduced, as a shell. Therefore, by coating Bi with a lower melting point with Te having a higher melting point, during sintering, the core Bi can be melted first, and then the shell Te can be melted. Therefore, the melting out of Bi can be reduced, and it can be obtained stably. Uniform alloy composition (for example, Bi ₂ Te ₃ ).

<About step (a)>

The method of the present invention for manufacturing core-shell metal nano particles includes introducing a first metal salt solution into a first flow path of a flow-type reaction device, and applying a plasma to the first metal salt solution in the first flow path to A step of obtaining a solution containing metallic nano particles of a first metal.

The first metal salt solution is a salt containing a first metal and a solvent, and is preferably substantially composed of a salt of a first metal and a solvent. Here, the expression "substantially made of a salt of a first metal and a solvent" means that, in addition to the salt of the first metal and a solvent, additives such as a dispersant are not actively contained.

As the first metal, any metal can be used, such as Semi-metal or non-transition metal (typical metal) such as Al, Ge, Sn, Sb, Te, Pb, or Bi, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Ag, Pt, or Au, etc. Transition metals, and combinations of these.

As the salt of the first metal, any metal salt can be used. Examples of the metal salt include mineral acid salts such as hydrochloride, nitrate, phosphate, sulfate, or hydrofluoride, carbonates, borate, silicate, or chromate. , Stearates, laurates, ricinoleates, or caprylates, or metal complexes such as ammonia complexes, cyano complexes, halogen complexes, or hydroxyl complexes组合。 The compound.

The solvent is not particularly limited as long as it is a salt capable of dissolving the first metal. Examples of the solvent include water or organic solvents. Examples of the solvent include alcohols such as ethanol, methanol, and isopropanol, heptane, hexane, and nonane. And the like, or aromatic hydrocarbons such as benzene, toluene, and xylene.

The concentration of the first metal salt solution in the first metal salt solution can be arbitrarily set based on the electric power of the plasma, or the particle size distribution of the desired metal nanoparticle.

As a method for introducing the first metal salt solution into the first flow path of the flow-type reaction apparatus, any method can be used, such as pumping or conveying by a cylinder.

The flow rate (mL / min) of the first metal salt solution is based on the cross-sectional area of the first flow path or the applied electric power of the plasma, etc., so as to obtain the desired particle size, particle size distribution, productivity, etc. of the metal nanoparticle. The method can be arbitrarily set.

Applied as a first metal salt solution in a first flow path The plasma method can be performed, for example, by applying a voltage to at least one of the electrodes installed in the first flow path to generate a plasma between the electrodes and passing a first salt solution through the electrodes.

The waveform of the applied voltage is not particularly limited, and examples thereof include a DC voltage, an AC voltage, and a pulse voltage.

The lower limit of the applied voltage is not particularly limited as long as it allows plasma to be generated between the electrodes, and it also varies depending on the particle size of the desired metal nanoparticle. For example, it can be set to 0.5 kV or more. It is 1.0 kV or more.

The upper limit of the applied voltage may be arbitrarily set, and may be, for example, 100 kV or less, and preferably 2.0 kV or less.

The lower limit of the applied power can be arbitrarily set based on the oxidation-reduction potential of the selected metal, as long as the selected metal can be precipitated, it is not particularly limited.

Herein, the redox potential in the present invention means a relative electrode potential (V) determined with respect to a standard hydrogen electrode.

For example, when the first metal is Bi, the oxidation-reduction potential of Bi ³⁺ is about 0.3172 V, which is the lower limit of the applied power, for example, 100 W or more, and preferably 140 W or more.

For example, when the first metal is Te, the oxidation-reduction potential of Te ⁴⁺ is about 0.5213 V, which is the lower limit of the applied power, for example, 30 W or more, and preferably 50 W or more.

The upper limit of the applied electric power can be arbitrarily set depending on the particle diameter of the desired metal nano particles, and is preferably 10 kW or less, for example. Can be set to 500W or less.

The particle diameter of the metal nano particles of the first metal can be set to an arbitrary particle size according to the use of the core-shell metal nano particles, and the lower limit of the particle size is, for example, 0.1 nm or more, and preferably 10 nm. Above, the upper limit of the particle diameter may be 500 nm or less, and preferably 30 nm or less.

In the present invention, the particle diameter is determined by observing a scanning electron microscope (SEM), a transmission electron microscope (TEM), and the like, and directly measuring the equivalent particle diameter of the projection area circle based on the photographed image, and analyzing the The number average primary particle size can be obtained from a particle group formed by a set number of 100 or more.

<About step (b)>

The method of the present invention for manufacturing core-shell type metal nano particles includes introducing a second metal salt solution into a second flow path of a flow-type reaction device so as to converge with a solution of metal nano particles containing a first metal And forming a mixed solution, applying a plasma to the mixed solution, and coating the metal nano particles of the first metal with the second metal.

The second metal salt solution is a salt containing a second metal and a solvent, and is preferably substantially composed of a salt of a second metal and a solvent. Here, the expression "substantially made of a salt of a second metal and a solvent" means that it does not actively include additives such as a dispersant other than the salt of the second metal and the solvent.

As a second metal, a salt of the second metal, and a solvent, And the concentration of the salt of the second metal in the second metal salt solution can be set to be the same as the description about the "first metal".

The introduction of the second metal salt solution into the second flow path and the flow rate may be the same as the description of the “first metal salt solution”.

The confluence of the first flow path and the second flow path is to mix a solution containing nano particles of the first metal and a second metal salt solution, as long as a mixed solution can be formed, and it can be performed in any state. For example, mixing may also use a mixing device to promote mixing.

Hereinafter, the flow path through which the mixed solution in which the first flow path and the second flow path meet is simply referred to as a "third flow path".

As a method of applying a plasma to the mixed solution, for example, by applying a voltage to at least one of the electrodes provided in the third flow path, the plasma is generated between the electrodes, and the mixed solution is passed between the electrodes.

The voltage and power of the plasma in step (b) can be the same as those described in step (a).

The particle diameter of the core-shell metal nanoparticle can be set to any particle size depending on the application, and the lower limit of the particle size is, for example, 0.1 nm or more, preferably 20 nm or more, as the upper limit of the particle size. It can be 500 nm or less, and preferably 30 nm or less.

《Flow type reaction device》

The flow-type reaction device of the present invention has a "first flow path", The second flow path and the third flow path are formed by converging the first flow path and the second flow path. In addition, the first flow path has at least one electrode pair that generates a plasma, and the third flow path has at least one electrode pair that generates a plasma.

The so-called "flow type reaction device" generally refers to a device that allows a raw material solution to continuously flow in a flow path, can perform operations such as reaction and mixing in the flow path, and can continuously produce products.

In the present invention, the size of the flow-type reaction device is not particularly limited. For example, when the cross-sectional area of the flow path is converted into a circle of the same area, the upper limit of the equivalent diameter of the flow path is, for example, 10 mm or less, and preferably 3 mm or less. In particular, the upper limit of the equivalent diameter of the flow path in the part where the plasma is applied is, for example, 10 mm or less, preferably 1 mm or less. In this case, the plasma can be applied more uniformly to the solution passing through.

The lower limit of the equivalent diameter of the flow path is, for example, 1 μm or more, and preferably 100 μm or more.

The flow-type reaction apparatus is not particularly limited, and examples thereof include those generally referred to as "microreactors".

The electrode pair may be any electrode as long as it can generate a plasma by applying a voltage, and any electrode can be used. Examples of the material of the electrode pair include tungsten, copper, chromium, and graphite.

For details of the other components, please refer to the description of the method of the present invention for manufacturing core-shell metal nano particles.

Core-Shell Metal Nanoparticles

The core-shell type metal nanoparticle of the present invention is a core-shell type metal nanoparticle obtained by a method including the following steps: (a) introducing a first metal salt solution into the first part of a flow-type reaction device; The first flow path is a step of applying a plasma to the first metal salt solution in the first flow path to obtain a solution containing the metal nano particles of the first metal; and (b) introducing the second metal salt solution into the flow The second flow path of the type reaction device is made to converge with a solution of metal nano particles containing the first metal into a mixed solution, and a plasma is applied to the mixed solution to cover the metal of the first metal with the second metal. Steps of Nano Particles.

With the core-shell metal nanoparticle of the present invention, a product with extremely high purity (for example, a catalyst, or a thermoelectric conversion material, etc.) can be manufactured, and thus high characteristics (for example, high catalyst function, or high Thermoelectric conversion characteristics).

<Impurity element>

In the present invention, the so-called impurity element included in the core-shell type metal nanoparticle refers to an element that is not intentionally included in the composition of the core-shell type metal nanoparticle. Therefore, additives that are added so as to be intentionally included in the final core-shell metal nanoparticle are not considered as impurity elements.

The impurity element is not particularly limited, and examples include metals such as alkali metals, alkaline earth metals, or transition metals derived from reducing agents and / or dispersants, base metals such as boron, aluminum, or silicon, semimetals, carbon, and nitrogen. , Metal, phosphorus, or sulfur.

"Alloy particles"

The alloy particles of the present invention are alloy particles obtained by alloying the core-shell type metal nano particles of the present invention.

By using the alloy particles of the present invention, extremely high-purity products (for example, catalysts, or thermoelectric conversion materials, etc.) can be manufactured, and thus, products having high characteristics (for example, high catalyst function or high thermoelectric conversion characteristics) can be obtained. product.

Not limited to theory, but with the growth of alloy particles due to alloying, impurity elements such as Na will be driven out of metal nano particles, so it is thought that the concentration of impurity elements can be made as described above .

As the method of alloying, any method can be used, and examples include heat treatment such as hydrothermal synthesis.

Hydrothermal synthesis can be performed by any method, for example, placing core-shell type nano particles and water in a closed container such as an autoclave, and heating can be performed while the container is closed.

The alloying temperature can be arbitrarily set as long as it can alloy at least a part of the core-shell metal nanoparticle.

For example, in the case of core-shell type nano particles containing Bi and Te, the lower limit of the alloying temperature is, for example, 150 ° C or higher, preferably 250 ° C or higher, and the upper limit is, for example, 400 ° C or lower. It may be set below 300 ° C.

《Thermoelectric Conversion Materials》

The thermoelectric conversion material of the present invention is a thermoelectric conversion material obtained by sintering the core-shell metal nanoparticle of the present invention or the alloy particle of the present invention.

With the thermoelectric conversion material of the present invention, the possibility of loss of thermoelectric conversion characteristics due to an impurity element that has been considered to be sufficiently removed in the past can be minimized, and high thermoelectric conversion performance can be obtained.

The sintering method can be performed by any method, for example, core-shell metal nano particles or alloy particles are formed by compression molding or the like in advance, or they are arbitrarily placed in a mold by sintering. It can be heated in a furnace.

The sintering temperature may be any temperature as long as the particles can be bonded to each other and scattering of the constituent elements is allowed, and can be arbitrarily set.

For example, in the case of core-shell metal nano particles or alloy particles containing Bi and Te, the lower limit of the sintering temperature is, for example, 300 ° C or higher, preferably 400 ° C or higher, and the upper limit is, for example, 550. It is preferably at most 450 ° C. or lower.

The sintering can be performed in air, and can optionally be performed in an inert gas such as nitrogen or argon.

When sintering, arbitrarily apply pressure to promote sintering.

[Example]

The present invention is described in more detail below, but the present invention is not It is not limited to the following examples.

<< Example 1 >>

In Example 1, a core-shell metal nanoparticle of Te-Bi with Te as the core and Bi as the shell was produced, and this was alloyed to produce Bi ₂ Te ₃ alloy particles, and Bi ₂ Te ₃ The alloy particles are sintered to produce a Bi ₂ Te ₃ thermoelectric conversion material.

FIG. 1 is a schematic view showing a method of the present invention for manufacturing core-shell type metal nano particles and an exemplary embodiment of a flow-type reaction apparatus of the present invention.

As the first metal salt solution (20), a solution containing 0.214 g of TeCl ₄ in 100 mL of an ethanol solvent was used.

As the second metal salt solution (30), a solution containing 0.170 g of BiCl ₃ in 100 mL of an ethanol solvent was used.

In the procedure (a), as shown in FIG. 1, a pump (P1) is used to introduce the first metal salt solution (20) into the first flow path (11) of the flow-type reaction device (10) at 10 mL / min. in.

For the tungsten electrode pair (14a) on the first flow path (10), a plasma was generated with a power of 50 W plus a voltage of 1.5 kV, and the first metal salt solution (20) was passed to apply the plasma. Thereby, a solution of Te-containing metal nano particles as a core was obtained.

In the procedure (b), as shown in FIG. 1, the pump (P2) is used to introduce the second metal salt solution (30) into the second flow path (12) of the flow-type reaction device (10) at 10 mL / min. ), It is mixed with the metal nano particles containing Te The daughter solution converges and becomes a mixed solution. The flow rate of the mixed solution in the third flow path (13) was 20 mL / min.

For the tungsten electrode pair (14b) provided in the third flow path (13), a voltage of 140 W plus a voltage of 1.5 kV was used to generate plasma between the electrodes, and the mixed solution was passed to apply electricity to the mixed solution. Pulp. Thereby, 200 mL of a Te-Bi core-shell metal nanoparticle solution (40) containing Te-Bi metal nanoparticle was coated with Bi as a core at 20 mL / min.

The obtained solution was filtered and the core-shell metal nano particles of Te-Bi were taken out, washed with ethanol, washed with water, washed with ethanol again, and dried to prepare about 12 g of Te-Bi. Core-shell metal nano particles.

The obtained Te-Bi metal nano particles were hydrothermally synthesized at 270 ° C for 10 hours to alloy them, thereby obtaining an aqueous solution containing Bi ₂ Te ₃ alloy particles. The obtained aqueous solution was filtered and the Bi ₂ Te ₃ alloy particles were taken out. This was washed with ethanol, washed with water, washed with ethanol again, and dried to produce Bi ₂ Te ₃ alloy particles.

Finally, the obtained Bi ₂ Te ₃ alloy particles were sintered at 400 ° C. in an Ar atmosphere to produce a sintered body of the Bi ₂ Te ₃ thermoelectric conversion material.

<< Example 2 >>

In Example 2, as the first metal salt solution (20), a solution containing 0.170 g of BiCl ₃ was used in a 100 mL ethanol solvent; and as the second metal salt solution (30), a solution containing 100 mL of an ethanol solvent was used. There was a solution of 0.214 g of TeCl ₄ .

Moreover, in Example 2, except that the power of the plasma in the program (a) was set to 140W and the power of the plasma in the program (b) was set to 50W, the same operation as in Example 1 was performed to produce Bi- Te core-shell metal nano particles are about 12 g.

<< Reference Example 1 >>

In Reference Example 1, a core-shell type metal nanoparticle of Te-Bi was produced by the following procedure using a batch-type solution plasma method.

As shown in FIG. 2, Reference Example 1 uses a solution containing 0.170 g of BiCl ₃ and 0.214 g of TeCl _{4 in} a 200 mL ethanol solvent as a raw material solution (51).

The raw material solution (51) was placed in a batch reaction device (50), and while stirring with a stirrer (53), a voltage of 1.5 kV was applied to the tungsten electrode pair (52) with a power of 50 W. A plasma is generated to apply a plasma to the raw material solution (51). With this, Te, which is easier to be reduced, starts to precipitate first.

While continuously stirring and applying a plasma at 50 W, the decrease in transmittance of the solution as the nano particles of Te grew was tracked by a visible ultraviolet spectrophotometer (UV-vis). When the transmittance of the solution is reduced to 3% of the initial transmittance before the application of the plasma, the electric power is switched to 140W, so that Bi, which is less easily reduced, is precipitated on the nano particles of Te. The time for switching the power is about 30 minutes after the start of plasma application.

While continuously stirring and applying the plasma at 140W, by A visible ultraviolet spectrophotometer (UV-vis) can be used to track the decrease in transmittance of the solution as the Bi shell grows. When the transmittance of the solution decreases to 25% of the initial transmittance before the application of the plasma, the application of the plasma is stopped. The time to stop the application of plasma is about 100 minutes after the application of plasma.

The obtained solution was filtered and the core-shell metal nano particles of Te-Bi were taken out, washed with ethanol, washed with water, washed with ethanol again, and dried to prepare about 12 g of Te-Bi. Core-shell metal nano particles.

Comparative Example

In the comparative example, NaBH _{4 was} used as a reducing agent to produce composite metal nano particles of Bi and Te, and this was alloyed to produce Bi ₂ Te ₃ alloy particles, and this was sintered to produce Bi ₂ Te ₃ thermoelectric Conversion material.

As a raw material solution, a solution containing 0.170 g of BiCl ₃ and 0.214 g of TeCl ₄ in 100 mL of an ethanol solvent was used. As the reducing agent, a reducing agent solution containing 0.218 g of NaBH ₄ in 100 mL of an ethanol solvent was used.

The raw material solution was placed in a container, and a reducing agent solution containing NaBH ₄ was added while stirring to precipitate the composite metal nano particles of Bi and Te.

The obtained solution was filtered and the composite metal nano particles of Bi and Te were taken out, washed with ethanol, washed with water, washed with ethanol again, and dried to prepare composite metal nano particles of Bi and Te.

The obtained composite metal nano particles of Bi and Te were hydrothermally synthesized and alloyed at 270 ° C for 10 hours to obtain an aqueous solution containing Bi ₂ Te ₃ alloy particles.

The obtained aqueous solution was filtered and the Bi ₂ Te ₃ alloy particles were taken out. This was washed with ethanol, washed with water, washed with ethanol again, and dried to produce Bi ₂ Te ₃ alloy particles.

Finally, the obtained Bi ₂ Te ₃ alloy particles were sintered at 400 ° C. in an Ar atmosphere to produce a sintered body of the Bi ₂ Te ₃ thermoelectric conversion material.

"Evaluation"

As shown in FIG. 3, in Examples 1 and 2, 200 mL of a solution containing core-shell metal nano particles can be obtained in 10 minutes, and 12 g of core-shell metal nano particles can be obtained from the solution. In comparison, in Reference Example 1, the time required for the same amount of 12 g of core-shell metal nano particles to be precipitated was about 100 minutes. From this result, it can be seen that the method and apparatus of the present invention are excellent in productivity.

The STEM image of the Te-Bi core-shell metal nanoparticle obtained using Example 1 is shown in FIG. 4 (a), and the result of EDX analysis along the white line in FIG. 4 (a) is shown in FIG. 4 (b). The STEM image of the Bi-Te core-shell metal nanoparticle obtained using Example 2 is shown in FIG. 5 (a), and the result of EDX analysis along the white line of FIG. 5 (a) is shown in FIG. 5 (b).

As shown in FIG. 4 and FIG. 5, by the method of the present invention, whether it is Bi or Te, it can be made into a core, or it can be made into shell. In contrast, by the method of Reference Example 1, when high power is used from the beginning, both Bi and Te are precipitated, so Bi cannot be made a nucleus. Therefore, it can be known that the design freedom of the core and shell of the method and device of the present invention is high.

The curve in FIG. 6 indicates the concentration (ppm) of Na as an impurity element contained in the “composite metal nano particles of Te and Bi” manufactured using the “Comparative Example” as indicated by the dotted line on the left. In the graph of FIG. 6, the dotted line is on the right side, and represents the concentration (ppm) of Na as an impurity element contained in the “alloy particles” obtained by alloying the composite metal nano particles of Te and Bi produced in the “Comparative Example”. ).

As shown in FIG. 6, in the comparative example “Te and Bi composite metal nano particles”, Na as an impurity element can be detected to exceed 300 to 4000 ppm, and “alloy particles” can detect Na as an impurity element. It is more than 5ppm ~ 200ppm. In contrast, the core-shell type nanoparticle produced in Example 1 was used, and since no additives such as a reducing agent and a dispersant were used, Na was not detected as an impurity element.

<< Reference Examples 2 and 3 >>

In the above examples, examples of core-shell metal nano particles including Bi and Te for producing thermoelectric conversion materials have been described, but the present invention is not limited thereto. Those with ordinary knowledge in this technical field may refer to the above disclosure and the description of the following reference examples, and may use other metal salts as the first metal salt or the second metal salt.

<Reference Example 2>

The batch-type solution plasma method was used to produce core-shell type nano particles of Au-Cu suitable for use as a catalyst metal in the following procedure.

As a raw material solution, 200mL ethanol solvent system used to contain 1.2mmol of tetrachloroauric (III) acid (HAuCl ₄ .4H ₂ O), 4.8mmol of copper acetate _{(II) (Cu (CH 3} COO) 2 .H 2 O), and 5 mmol of NaI solution.

In the same manner as in Reference Example 1, Au-Cu core-shell nano particles were produced by using an applied voltage of 1.5 kV and switching the power of the plasma from 50 W to 140 W.

FIG. 7 is a TEM image showing the obtained metal nano particles. It can be seen that they are core-shell metal nano particles that form Au-Cu.

<Reference Example 3>

The batch-type solution plasma method was used to produce core-shell type nano particles of Au-Co suitable for use as a catalyst metal in the following procedure.

As a raw material solution, 200mL ethanol solvent system used to contain 1.2mmol of tetrachloroauric (III) acid (HAuCl ₄ .4H ₂ O), 4.8mmol of cobalt acetate _{(II) (Co (CH 3} COO) 2 .4H 2 O), and 5 mmol of NaI solution.

In the same manner as in Reference Example 1, Au-Co core-shell nano particles were produced by using an applied voltage of 1.5 kV and switching the power of the plasma from 50 W to 140 W.

FIG. 8 is a TEM image showing the obtained metal nano particles image. It can be seen that they are core-shell metal nano particles that form Au-Co.

Anyone with ordinary knowledge in this technical field can know that both Reference Example 1 and Example 1 are core-shell type metal nano particles that can produce Te-Bi, and described in Reference Example 2 A solvent containing tetrachlorogold (III) acid (HAuCl ₄ .4H ₂ O) in ethanol was set as the first metal salt solution, and copper (II) acetate (Cu (CH ₃ COO) ₂ ) was contained in ethanol. The present invention can be implemented by using a solvent of H ₂ O) as the second metal salt solution. Similarly, anyone with ordinary knowledge in the technical field will know that from the description in Reference Example 3, ethanol is used as a solvent containing tetrachlorogold (III) acid (HAuCl ₄ .4H ₂ O). as a first metal salt solution, containing ethanol as cobalt (II) acetate _{(Co (CH 3 COO) 2} .4H 2 O) solvent to a second metal salt solution, the present invention can be implemented.

Claims (7)

A method for manufacturing core-shell type metal nano particles, which comprises the following steps: (a) introducing a first metal salt solution into a first flow path of a flow-type reaction device; A step of applying a plasma to the metal salt solution to obtain a solution of the metal nano particles containing the first metal; and (b) introducing the second metal salt solution into the second flow path of the flow-type reaction device, The solution of the metal nanoparticle solution of the first metal converges on the third flow path and becomes a mixed solution. The step of applying a plasma to the mixed solution and coating the metal nanoparticle of the first metal with the second metal is described above. The redox potential of one metal is lower than the redox potential of the aforementioned second metal. The method of claim 1, wherein the first metal is Bi and the second metal is Te. The method according to claim 1, wherein the equivalent diameter when the cross-sectional areas of the first flow path and the third flow path are converted into circles of the same area is 1 μm to 10 mm. A flow type reaction device includes: a first flow path; a second flow path; and a third flow path. The third flow path is formed by converging the first flow path and the second flow path. The first flow path has at least one electrode pair generating a plasma, and the third flow path has at least one electrode pair generating a plasma; and a first container for storing a first metal salt solution is installed upstream of the first flow path A second container for storing a second metal salt solution is installed upstream of the second flow path. The redox potential of the first metal is lower than the redox potential of the second metal, and the second metal is covered with the second metal. Metal nano particles of the first metal obtained through the first flow path. The flow-type reaction device according to claim 4, wherein the equivalent diameters of the first flow path and the third flow path to which the plasma is applied are 1 μm to 10 mm, respectively. A method of manufacturing a thermoelectric material, comprising: manufacturing a core-shell type metal nanoparticle by the manufacturing method of any one of claims 1 to 3; and sintering the nanoparticle. A method for manufacturing a thermoelectric material, comprising: manufacturing a core-shell metal nanoparticle by the manufacturing method of any one of claims 1 to 3; alloying the nanoparticle to obtain alloy particles; and sintering The alloy particles.